practice: untagged problems — target optional, topic sessions work (owner round 12)

A session whose material is not in the map is a SUPPORTED mode now:
target is optional (absent = untagged problem, poses freely), an
explicit [] heals away at the seam (the record's canonical form is
tagged-or-absent), and when targets ARE present the round-9 law is
unchanged — whole-graph validation, names normalized to ids. The
rejection message teaches the exit ('omit target entirely'), so a
model inventing targets for an off-graph topic converges to omitting
on the retry; omitting always works, no brick is possible. Prompt
carries the clause: material in the map → ids; not → omit, problems
stand on their own.

src/agent/prompts/practice/00-base.md

@@ -23,7 +23,7 @@ He has to be able to trust you completely; one confidently-delivered hallucinati
 
 Two tools, and everything else is conversation. The mechanics only — the method (when to pose, what to pose, how to respond) is the doctrine's.
 
-**`problem`** — poses one problem. The statement field is what the learner sees (markdown and LaTeX render); the modelSolution is your private commitment, hidden from the learner until you conclude. The target field takes the proposition ids from the session context — ids are how every problem is traced to the propositions it exercises. The call ends the turn. The learner then works in a thinking field; each submission arrives back as a *Your thinking* message. Whether it claims an answer is IN THE TEXT, not declared anywhere — read it:
+**`problem`** — poses one problem. The statement field is what the learner sees (markdown and LaTeX render); the modelSolution is your private commitment, hidden from the learner until you conclude. When the session's material lives in the map, target takes the proposition ids from the session context — ids are how every problem is traced to the propositions it exercises. When it does not (a topic with nothing in the map), omit target entirely: problems stand on their own, untagged. The call ends the turn. The learner then works in a thinking field; each submission arrives back as a *Your thinking* message. Whether it claims an answer is IN THE TEXT, not declared anywhere — read it:
 
 - Working trace (mid-work, stuck, or asking for a nudge): the hint ladder applies, one rung per request. A trace may span several submissions.
 - A claimed answer: extract the moves, deliver the verdict. A claim is not a progress signal.